1,1′‐Bis(diphenylphosphino)bicyclopropyl: Synthesis, Properties, Precursors, Derivatives, and Metal Complexes

Abstract

The title compound 4 has been prepared from readily available 2,3‐bis(diphenylphosphinyl)‐1,3‐butadiene (1) through double cyclopropanation using Me2S(CH2)O to give 1,1′‐bis(diphenylphosphinyl)bicyclopropyl (2), followed by reduction using HSiCl3/NEt3. Addition of sulfur to compound 4 yields the disulfide 5, and reaction with tetrahydrofuran – borane affords the 1:2 adduct with BH3 (6). Quaternization reactions with MeI or CH2I2 give the double quaternary salts 7 and 8, respectively. Single dehydrohalogenation employing nBuLi converts 8 into the cyclic semiylide salt 9. 4 is an excellent ligand for lowvalent late transition‐metal cations. With PdI2 the 1:1 complex LPdI2 (10, with L = 4), and with [(CO)2RhCl]2 the ionic 2:1 complex L2Rh+Cl‐ (11) are obtained. Experiments with (CO)AuCl yield the 1:2 complex L(AuCl)2 (12), X‐ray structure analyses were performed with single crystals of the disulfide 5, as well as the rhodium(I) and gold(I) complexes 11 and 12. 5 has a conformation between s‐cis and s‐trans with the PS functions pointing away from each other at opposite ends of the molecule. By contrast, in the gold(I) complex the ligand approaches an s‐cis conformation, and through rotations about P–C and C–C bonds – as referred to the conformation of 5 – the metal atoms are brought into close contact: Au…Au = 3.085 Å. Through temperature‐dependent NMR investigations of compounds 5 and 12, and by comparison with values calculated or experimentally determined for related bicyclopropyl compounds (available in the literature), the energy of the Au…Au attraction has been estimated to be ca. 6 kcal/mol. Compound 11 features a square‐planar, double‐chelate cation.

The fascinating implications of new results in gold chemistry

“…It is obvious from the formulae also, that the conformations adopted in the complexes contradict electrostatic reasoning, since units of equivalent charge or polarization are brought together -an unknown phenomenon for standard situations with other dinuclear complexes of closed-shell metal atoms. The values arrived at in the three types of complexes are in the order of 6-8 kcal/mole [54][55][56]. This result confirms the classification of the Au•••Au attraction as à weak force', compared best in magnitude perhaps with hydrogen bonding, or with solvation in weakly polar media.…”

“…In 1:1 complexes, these conformations are retained, which is proof that phosphine coordination in principle is not necessarily associated with conformational changes. In the 2:1 complexes, however, such a change, which brings the gold atoms into proximity, is consistently observed (Figures 8b, 9, 10), [54][55][56].…”

“…A series of tri-organophosphoniumbis(diphenylphosphino)methylides [54] and the di-tertiary phosphines derived from butadiene [55] and bicyclopropyl [56] adopt -as free ligands -ground state conformations which do not allow metal-metal contacts in their dinuclear complexes with AuCI. The former have a syn/anti conformation relative to the ylidic function (Figure 8a), and the latter two adopt the single-trans conformation (butadiene) or a form close to this extreme (bicyclopropyl) [57].…”

“…Through adoption of a near-cis conformation, not observed with the free ligand, An--Au interactions are established[56].…”

Three-Membered-Ring-Based Molecular Architectures

“…249 The bis-(diphenylphosphinoxy) derivative 354 was reduced to the 1,1′-bis(diphenylphosphinyl)bicyclopropyl, which was used as a ligand in various low-valent metal complexes. 255 The latter was also transformed into the corresponding sulfur and borane adducts.…”